The spliceosome is key in the creation of proteins from DNA. This ribonucleoprotein complex acts to remove introns from pre-mRNA transcripts, providing a template for accurate translation. Recent studies have shown that proteins involved in spliceosome assembly are targeted by antitumor drugs. In addition, the spliceosome has been used in gene therapy by promoting trans-splicing reactions. These reactions can act to reprogram RNA to repair an RNA transcript from a faulty gene, which, when translated could otherwise lead to cancer. Gene therapy could also be used to add an RNA transcript, which could cause cell death. Using enzymes in Drosophila as a model for human spliceosome proteins, this project aims to gain structural insight into the steps involved in spliceosome formation and the reactions involved in intron cleavage. Using X-ray crystallography, the structures of several proteins involved in spliceosome assembly will be solved. Drosophila Pelement somatic inhibitor (PSI), a protein involved in alternative splicing, will be determined in complex with substrate RNA. In addition, a complex of U2AF, SF1 and RNA, critical for recruitment of the snRNPs necessary for proper spliceosome assembly, will be solved to gain insight into the requirements for spliceosome formation. The results of this work would provide details necessary for the discovery of new antitumor agents, and would further research involving the use of spliceosome components as targets for cancer therapy.